It's the largest Civilization experience you will find on the PC at the moment. If you're a fan then you've probably got it and lost half a year to it already, if you're not then prepare to taste one of the most addictive forces in the universe.
For newcomers the concept is to take control of a civilization, build them up in turn based gameplay, and then take over the world by either force, culture, diplomatic means, technological advancements or (if you don't disable it) reaching the end of a set number of turns as the most successful civilization on the map. Beyond the Sword is the second (and best) expansion pack for the Civilization IV game, and boasts the title of the biggest expansion pack for a Civilization game ever. Why? 10 new civilizations, 16 new leaders, 12 new scenarios, 25 new units, 18 new buildings, 5 new wonders and that's before you even consider the amount of external mods out there as well. It's also the biggest Civ expansion ever because it actually pretty much includes everything from the last expansion pack Warlords (which is super if you didn't buy Warlords but something of a kick in the teeth if you did).
The expansion pack is aimed pretty much entirely at the post gunpowder stages of the game, which makes for a longer and more in depth journey to the end. In particular the inclusion of corporations (more or less a capitalistic version of religions), the Apostolic Palace (which if I'm honest is more of a late game religious novelty), random game events (such as quests like build a bunch of Colosseums so we can have a sports event) and a further developed espionage aspect do add new dimensions to the game, if only a little in some cases. If you're looking for a new dynamic way to play however then you need look no further than the new mods and scenarios on offer. Here is where the expansion pack really starts to shine.
Amongst Beyond the Sword's new mods and scenarios, the one which will probably be used the most is the 'Next War' mod. Boot this one up and what you'll get is a normal game of Civ with all the Beyond the Sword usual additions, only this time once you've reached the end of a normal game it keeps going. Suddenly you've gone from tanks and paratroopers to Dreadnaughts and clone armies. You can build biological weapons or take on the enemy with cyborgs and national mech units. The only criticism I can find with this addition is that it doesn't go far enough. I remember times of old with Civ spin-offs such as Call to Power where you could create underwater cities and space colonies. By comparison Next War is a little bit limited in the future tec it offers.
If you're looking for an entirely different way to play the game however then you'll feel right at home with scenarios such as Final Frontier and Age of Ice. In Final Frontier the whole game is changed as you battle for solar systems in space, whilst in Age of Ice you use magic and fight dragons and Gods. Otherwise you can play a completely historically accurate game of Civ in Rhye's and Fall of Civilization, or battle as the Allies or Axis with whole new war time technologies and units in World War II: Road to War.
As you can see, this is no ordinary expansion pack; and dare I say it goes even further. A little venture online in the right places and you will find reams more scenarios and mods by third parties. Some of these are nothing short of excellent. If you stumble upon the 'Fall From Heaven 2' mod for example you are in for a treat. This is the squeal to the Age of Ice scenario that comes included with Beyond the Sword, and as soon as you open the mod it becomes very apparent that Age of Ice was just a teaser. In this game angelic, demonic and crazy carnival gone wrong civilizations, vast upgrades of magic, and legionary heroes, dragons and mythical creatures are all common place. This mod alone is effectively a brand new game of civilization. Also the 'Thomas' war' mod adds to Next War with vast amounts of even more units, projects, technologies, buildings etc (still no underwater cities, national mind control wonders or Alien life projects though).
On it's own Beyond the Sword gets a well deserved 9 out 10. It adds new everything to the game to the point where it's verging on becoming a Civilization V. In particular the new scenarios and mods will keep you hooked for literally weeks. Some of the new enhancements are more novel than revolutionary, but none the less they still add to an overall well rounded package. If the external third party content found on the internet is included however then it bumps up to a 10 out of 10. This content will keep you hooked for months, and even if you think you've played them all, all of a sudden you'll notice a Star Wars mod or something equally ridiculous and incredibly addictive. This expansion pack has set a bar for expansion packs everywhere, and quite frankly I wouldn't be surprised if it's a long time before anything else comes even close.